Compression type refrigeration systems employ an evaporator which is supplied with low pressure refrigerant liquid. The low pressure refrigerant boils away or evaporates when supplied with heat from a medium to be cooled. The most common media which are cooled by such systems are streams of air, and streams of water or aqueous brines. The refrigerant vapor emitted from the evaporator is delivered by a pipe called a suction line to a mechanism which simultaneously acts as a vacuum pump to draw vapor from the evaporator and as a condensing device to restore the refrigerant vapor to a liquid condition so it can be reused in the evaporating part of the refrigerating cycle. The evacuating and condensing mechanism is called a condensing unit. The condensing unit has two major components. The evacuating device is most frequently a mechanical compressor driven by an electric motor. The compressor draws refrigerant vapor from the evaporator and compresses it and delivers it via a pipe to a condenser. The condenser condenses the hot refrigerant vapor to a refrigerant liquid by bringing it into heat exchange with a coolant. The most commonly employed coolants are air, employed in air-cooled condensers, water, employed in water cooled condensers and a mixture of air and water employed in so-called evaporative condensers.
The refrigerant liquid is then generally transmitted from the condenser to a holding tank called a receiver, where it is stored until needed by the evaporator. The refrigerant liquid when stored in the receiver generally has a temperature which is a few degrees cooler than the temperature at which it condensed called the saturated condensing temperature. The number of degrees which the refrigerant liquid is cooler than the saturated condensing temperature is called the subcooling or the degrees of subcooling. When the refrigerant liquid leaves the receiver it is in the form of liquid without any bubbles. However, if the subcooling is reduced to zero either by warming the refrigerant liquid those few degrees of subcooling or by lowering the pressure on the refrigerant liquid, bubbles, often called flash-gas, will form in the refrigerant liquid.
When the refrigerant liquid flows toward the evaporator from the receiver in a pipe called the liquid line, it is at high pressure. In order for the refrigerant liquid to evaporate and cool the fluid needing refrigeration, its pressure must be reduced. This pressure reduction is secured by passing the high pressure refrigerant liquid through a flow restrictor, also called an expansion device. Flow restrictors come in many forms. One is in the form of a length of tubing having a very small bore called a capillary tube. It is the form of restrictor most often used in domestic refrigerators, freezers and room air-conditioners. Another is in the form of a fixed orifice, frequently used in automotive air-conditioners. The form of restrictor most frequently employed in larger commercial or industrial refrigeration systems of the type toward which the present invention is primarily directed is a valve which senses both the pressure in the evaporator and the temperature at the refrigerant vapor outlet of the evaporator. This dual sensing valve is called a thermal expansion valve or TXV for short.
TXV's work best when the refrigerant liquid fed to them is free of bubbles. Such bubble-free liquid is also called clear liquid or "solid" liquid. Used in this sense, "solid" liquid is not frozen liquid but is simply refrigerant liquid which is free of bubbles.
Since the refrigerant liquid which is stored in the liquid receiver has only a few degrees of subcooling, it is not uncommon for the refrigerant liquid to reach the TXV inlet in a bubbling state. Expansion valves receiving bubbling refrigerant liquid tend to act erratically. Erratic TXV performance has a detrimental effect on evaporator capacity and therefore on overall system capacity.
Refrigeration systems frequently have their condensers and receivers located at ground level and their TXV and evaporators positioned at a much higher level. During the refrigeration cycle the liquid refrigerant flowing to the TXV and evaporator is exposed to severe loss of sub-cooling and therefore high likelihood of bubbling by the pressure loss caused by the flow of the liquid to a higher elevation and by the friction loss from the long run of piping and from the pressure-drop producing, and therefore sub-cooling reducing, valves and fittings which are positioned in the liquid line between the receiver outlet and the TXV.
In order to control these flash gas producing factors many costly design stratagems are employed. Among these are increasing the diameter of the liquid line, raising the condenser and receiver to a level near that of the TXV, oversizing all the pressure-drop producing flow elements or providing a suction-liquid heat exchanger. In some cases, it is so difficult to maintain a bubble-free supply of refrigerant liquid to the TXV that the TXV is deliberately oversized to allow a semblance of reasonable, though significantly degraded, performance with bubbles entering the TXV.
To overcome the tendency of refrigeration systems to deliver bubbling refrigerant liquid to their TXV so-called suction-liquid heat exchangers are frequently employed. These heat exchangers are installed in the system suction line. The piping is arranged to pass the vapor emitted from the suction outlet of the evaporator in heat exchange relation to the high pressure refrigerant liquid flowing from the receiver to the TXV. This heat exchange cools the refrigerant liquid and either condenses bubbles if any have formed in the liquid, or increases the degree of subcooling of the refrigerant liquid, thereby reducing the propensity of the refrigerant liquid to form bubbles. Unfortunately, suction-liquid neat exchangers have a series of disadvantages.
First, they introduce pressure drop in the suction line. Suction line pressure drop has the effect of reducing compressor capacity and therefore system capacity.
Second, they warm the suction vapor returning to the compressor from the evaporator with exactly the same number of heat units (Btus, calories etc) that are extracted from the refrigerant liquid flowing through the exchanger. The warmed suction vapor has dual negative effects: that of reducing the compressor capacity by presenting to the compressor warmed and therefor less dense refrigerant vapor to compress; and that of causing the high pressure vapor discharged by the compressor to be hotter than necessary. The higher the compressor discharge temperature, the thinner the compressor lubricant and the more likely the lubricant will suffer some thermolytic degradation resulting in shortened compressor life.
Third, suction-liquid heat exchangers fail to work when most needed. For example, when the TXV is in the mode of receiving a mixture of liquid and vapor, its flow capacity is so reduced that the evaporator cannot be fully flooded. Therefore the refrigerant vapor leaving the evaporator is warm and relatively ineffective to substantially cool the liquid/vapor mixture flowing toward the TXV.
Fourth, the suction-liquid heat exchanger cannot reduce the temperature of the refrigerant liquid flowing to the TXV to near the temperature of the fluid entering the evaporator, though the greatest improvement in evaporator capacity and stability of TXV performance is achieved with coldest liquid entering the TXV.
Finally, the suction-liquid heat exchanger is costly both to manufacture and to install.
It is against this background that I have conceived the present invention applicable to reversible compression type refrigeration systems, commonly known as heat pumps, which avoids all the problems described above. My improved heat pump system provides liquid subcooling without any suction line pressure drop.
My improved heat pump system does not contribute to any warming of the suction vapor enroute from the evaporator to the compressor.
My improved heat pump system works to sub-cool refrigerant liquid flowing to the TXV even when the evaporator is not fully flooded with refrigerant liquid.
My improved heat pump system cools the refrigerant liquid flowing to the TXV to a temperature close to the temperature of the fluid entering the one of the two main heat exchangers.
In addition, compared to a conventional heat pump system, my improved heat pump system has the following further advantages:
* It provides assurance of bubble-free refrigerant liquid at the TXV inlet. PA0 * It increases the heat pump system capacity. PA0 * In heating mode, where outdoor air is the heat source, it reduces the amount of surface participating in frost accumulation, thereby reducing both the time and energy required for a complete defrost. PA0 * In cooling mode with high sensible heat loads, ie. computer room applications, it provides increased sensible heat ratio of the evaporator. PA0 * It causes the evaporator to operate with higher entering temperatures of the fluid-to-be-cooled, thereby enhancing system coefficient of performance (COP). PA0 * It is adaptable to both finned coil heat exchangers for heating and cooling air and to shell or plate type heat exchangers for heating and cooling liquid. Though shell type heat exchangers are referred to, any heat exchanger capable of performing the required heat exchange is intended.
Although all the above factors apply in non-reversible refrigeration systems, the same factors are especially important in reversible refrigeration systems of the type most commonly employed both to heat a residence in winter and to cool it in summer.
It is the objective of the present invention to provide means for offsetting losses of sub-cooling and providing the other advantages and avoiding the disadvantages of the prior arrangements.